The present invention relates to medical ultrasonic imaging systems, and in particular to improved ultrasonic transmit pulses and transmit pulse generators for such systems.
Fundamental, harmonic, and subharmonic mode ultrasound imaging are all improved by controlling the bandwidth of the frequency spectrum of the transmitted ultrasonic pulse. This bandwidth is preferably limited to frequencies around the fundamental, and is preferably sharply reduced at specified harmonics and subharmonics of the fundamental.
One prior-art approach is to select the number of cycles of a carrier frequency that are transmitted in a square wave pulse burst. A greater number of carrier cycles in the burst corresponds to a narrower signal bandwidth. Another approach is to use a bipolar uniform square wave pulse train, which has less energy around the second harmonic frequency than does a unipolar uniform square wave pulse train of the same length.
Another approach is to use a carrier wave that is modulated by a gradually rising and gradually falling amplitude envelope. This approach requires an analog transmitter capable of generating output voltages at a large number of different levels. Cole U.S. Pat. No. 5,675,554, assigned to the assignee of the present invention, provides one example of this approach.
Dodd U.S. Pat. No. 5,833,614, also assigned to the assignee of the present invention, discusses several types of pulse width modulated (xe2x80x9cPWMxe2x80x9d) signals that can be used to approximate a carrier wave modulated by a gradually rising and gradually falling amplitude envelope. FIG. 3 of the Dodd patent shows a unipolar PWM signal having two amplitude levels. FIGS. 5 and 6 show bipolar PWM signals having three voltage levels (+V, 0V, xe2x88x92V). Note that the 0V level is held for some length of time within the duration of the pulse burst, greater than an instantaneous time.
There are a number of prior-art approaches to hardware for generating three level ultrasonic pulses. FIG. 1 shows one prior-art switched voltage supply that uses both a positive voltage power supply and a negative voltage power supply. Two FETs are used as switches to drive the load alternately with a positive-going and a negative-going waveform. When either FET is switched on, the output impedance is low, on the order of a few ohms. Normally, the waveforms generated by these types of transmitters are characterized by a 50% duty cycle and are held in the zero value state only before and after transmitting the pulse burst, not during the pulse burst. Both FETs are switched off in the zero value state, and in this state the output impedance of the FETs is high, approximately equal to the capacitive reactance of the two FETs. As shown in FIG. 1, a resistor is connected between the junction of the FETs and ground to lower the impedance in the zero value state. This shunt resistor brings the disadvantage that substantial power is wasted in the resistor when the transmitter is in the positive or negative output states. The transmitter of FIG. 1 also has the disadvantage that it requires both positive and negative high voltage power supplies. Furthermore, the gate drivers for all of the FETs are referenced to a voltage other than ground, which increases their circuitry complexity and cost. Low second-harmonic distortion performance requires that the generated waveform have excellent symmetry. This is difficult to achieve with the transmitter of FIG. 1.
FIG. 2 shows another prior-art transmitter that uses a transformer and N-channel FETs to form a bipolar, push-pull transmitter. In the transmitter of FIG. 2, the FETs are all operated as switches. The transmitter of FIG. 2 is capable of excellent waveform symmetry, and it uses ground referenced gate drivers for all of the FETs. Furthermore, the output impedance of the transmitter is low in either the positive voltage state or the negative voltage state. However, the output impedance increases to the value of R in the zero voltage state when both FETs are in the open-circuit state. If both FETs were placed in the closed-circuit state, the output voltage would be zero and the output impedance seen looking back into the transformer would be low, but the circuit would draw excessive current from the power supply as the current through the two halves of the center-tapped transformer primary would develop magnetic fluxes that cancel each other.
The prior-art transmitter of FIG. 3 is similar to that of FIG. 2, but in this case the FETs are operated as current sources, not as switches. The drives to the FETs are controlled to keep them operating in the pentode region of their drain family characteristic, where they have a high output impedance and can be considered as current sources. In this case, the output impedance of the circuit is set by the resistor R (neglecting the output capacitance of the FETs and the parasitic reactances of the transformers). The FETs can be driven at their gates or their sources. The circuit of FIG. 3 is power inefficient. When the output is in the positive or negative voltage state, the transmitter delivers wasted current to the resistor R. Power is also wasted in the FETs when they are operated as current sources, since it is necessary to maintain a drain-to-source voltage across them to keep them in the pentode region. Power is dissipated across the FETs because they have a current flowing through them at the same time that there is a voltage across them.
Thus, a need presently exists both for improved three-state transmit pulses that suppress energy at selected harmonic frequencies, and for improved transmit generators that are capable of generating three-state pulses efficiently, symmetrically and economically.
By way of introduction, the preferred embodiments described below use a new type of three-state, PWM transmit waveform that is the sum of two components. The first component is a two-state waveform at 0V and +V, and it has the desired fundamental frequency characteristics. The second component is a time-shifted, inverted version of the first component at voltages 0V and xe2x88x92V. By properly selecting the amount by which the second component is time-shifted relative to the first component, transmitted power at any selected harmonic frequency can be further reduced.
This improved three-state PWM waveform is preferably generated with a switched voltage source that provides low, substantially identical source impedances in each of the three states (+V, 0V, xe2x88x92V). Because the source impedance is low in all three states, the transmitter operates efficiently. Because the source impedance is constant in all three states, the transmitter is able to generate a symmetrical transmit waveform with excellent suppression of second harmonic components. One preferred embodiment described below achieves these important advantages using only N-channel switches controlled by gate drivers that are all referenced to ground. This embodiment has particularly low fabrication cost.
The switched voltage sources described below can be used to generate other waveforms, including waveforms described in the above-referenced Dodd patent. Additionally, these voltage sources can be adapted to generate waveforms with more than three voltage levels. The low source impedance of these switched voltage sources allows accurate generation of waveforms having two or more pulses of the same voltage polarity that are separated by a short-duration zero volt interval.
The foregoing paragraphs have been intended by way of introduction, and they are not intended to limit the scope of the following claims.